bird-mammal common ancestor
mturner at acpub.duke.edu
Sun Sep 11 20:22:09 EST 1994
In article <mwspitze-110994140419 at phar2.medsurge.hsis.uci.edu> mwspitze at uci.edu (matt spitzer) writes:
>> A question for evolutionary biologists:
>> What is the current thinking on the common ancestor of birds and mammals?
>So far I have received a number of informative responses. To summarize
>them, it is currently thought that mammals diverged prior to development of
>anything like a modern reptile (from which birds diverged much later).
>Does anyone disagree?
Basically right; the Synapsid- therapsid-mammal line is thought to be a
sister group to the other known Amniota (at least all living ones). The
main problem is that there is no definable group "reptiles" that means
anything more than "those amniotes that are not mammals or birds".
The basal branches of the mammal line are usually called the "mammal-like
reptiles". Among living groups, turtles appear to be a basal branch of the
other main line, then come a group with Sphenodon, lizards and snakes, and
finally the crocodilians as the living sister group to birds. (The tree gets
a lot busier if you include fossil groups). Birds are a branch of a branch
the dinosaurs; they belong to a larger group Archosauria including the crocs,
pterosaurs etc.; archosaurs and lizards, etc. comprise a larger group
Diapsida, and so forth. "Reptiles" should just be used as an informal term
for non-mammal, non-bird amniotes.
With regard to your earlier question about which living species is closest to
the common ancestor, it helps to keep in mind that all of these branching,
diverging lineages have been evolving along their own paths for exactly the
same amount of time and it is unlikely that any one of them would remain
primitive in all respects. ("primitive" = the characteristics of the msot
recent common ancestor of a group). It is misleading to pigeonhole whole
living organisms as either primitive or advanced--- (e.g., in many specific
skeletal characters most mammals are more primitive than frogs, turtles
or snakes). It is much more meaningful to try to analyze the organisms
character- by- character in order to reconstruct what the most recent common
ancestor at each branch point of the tree must have been like.
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